This application is a request for funds to purchase a JL Shepherd Mark I 68A irradiator that will be located in the Center for Health Sciences (CHS)/Public Health vivarium. This facility, the largest single vivarium on the UCLA campus, houses approximately 28,000 mice and 3,000 rats. However, investigators who are assigned space in the CHS/Public Health vivarium are unable to conduct experiments that require small animal irradiation because an irradiator is not located in this facility. While, as detailed in the proposal, there are seven irradiators on the UCLA campus, only two are appropriate, because of design or dose rate, for use with small animals such as mice and rats. Unfortunately, these two irradiators are located in small colonies that can not accommodate the needs of the large number of investigators who house their animals in the CHS/Public Health vivarium, and the units can not be relocated. Thus, obtaining funds to purchase the JL Shepherd Mark I, Model 68A irradiator is a priority. This particular model was chosen because its design and features are ideal for small animal irradiation. Although the unit itself is relatively small, it has a large chamber that will allow up to a dozen mice to be irradiated simultaneously. Further, several additional features such as an adjustable turntable, attenuators, and a collimeter allow considerable versatility in both the dose rate and the area of the animal to be irradiated. Locating the irradiator in the CHS/Public Health vivarium will greatly facilitate the work 'of the six investigators listed on this application. For example, three of the investigators use experimental bone marrow transplantation protocols in order to investigate the developmental and functional potential of hematopoietic populations. The ability to irradiate mice will also be of great benefit in studies of skeletal muscle disease progression and regeneration. The irradiator will be core instrumentation that will be made available to any qualified UCLA investigator.